Reflective multiplier phototube

ABSTRACT

A multiplier phototube includes a light reflective photocathode on a faceplate at one end and a reflective surface adjacent the electron multiplier at the other end to provide a tube of increased sensitivity and efficiency. The structure may be incorporated in an image dissector tube having an accelerating mesh adjacent the photocathode, a cylindrical anode between the mesh and multiplier and a scanning aperture plate at the entrance to the multiplier, with the reflective surface within the anode.

United States Patent 1 Plumeau 1 Nov. 6, 1973 [54] REFLECTIVE MULTIPLIER PHOTOTUBE 3,329,856 7/1967 Foote 313/95 X Inventor: Charles A. Plumeau, Plano, Tex 3,447,014 5/1969 Jordan 313/94 [73] Assignee: International Telephone and Primary Examiner-Roy Lake T g p Corporation, y, Assistant Examiner-Siegfried H. Grimm [22] Filed: Feb. 2 1972 Att0rneyC. Cornell Remsen, Jr. et a1.

[21] 'Appl. No.: 222,782 ABSTRACT A multiplier phototube includes :a light reflective phofi :Z tocathode on a faceplate at one end and a reflective 3 94 g surface adjacent the electron multiplier at the other I I e 0 care l end to provide a tube of increased sensitivity and efficiency. The structure may be incorporated in an image [56] References C'ted dissector tube having an accelerating mesh adjacent UNITED STATES PATENTS the photocathode, a cylindrical anode between the 2,440,735 5/1948 Cawein 313/67 mesh and multiplier and a scanning aperture plate at 2,972,691 2/1961 Kossel 313/94 the entrance to the multiplier, with the reflective sur- 3,229,105 1/1966 Mestwerdt et a1... 250/227 X f i hi h d 3,295,010 12/1966 Clayton 313/64 X 3,322,955 5/1967 Desvignes 250/213 VT X 4 Claims, 2 Drawing Figures 1 REFLECTIVE MULTIPLIER PHOTOTUBE BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention concerns photomultiplier tubes and particularly a novel structure using light reflection between opposite ends to provide a photomultiplier' tube of improved efficiency and sensitivity.

2. Description of the Prior Art Photomultiplier tubes are generally employed for the detection of light and for obtaining amplified electrical signals representative of the light. Various structures and materials have been used to improve the efficiency of electron emission from photocathodes in response to light and to increase the sensitivity of the tube and multiplier structures; One known structure for improving photocathode efficiency is found in U.S. Pat. No. 3,229,105, issued Jan. 1], I966, wherein areflective surface at the output end of an image intensifier tube reflects light onto the outer surface of a photocathode having an underlying reflective layer on the tube faceplate. This provides an intensified light output image with respect to the input light image. A similar reflective mirror surface has been used in a camera'tube for projecting light-onto a radiation sensitive target to be scanned by an electron beam, as shown in U.S. Pat. No. 3,322,955, dated May 30, 1967. These structures, however, are employed for different purposesthan photomultiplier tubes and do not provide an amplified electrical output signal representative of a light input signal as required in many applications.

SUMMARY OF THE INVENTION It is therefore the primary object of the present invention to provide a photomultiplier tube of increased sensitivity and to improve the efficiency of electron emissive photocathodes used therewith. A further object is to provide an improved image dissector tube incorporating anovel reflector structure.

This is achieved by a unique photomultiplier tube configuration employing a light reflective photocathode on the faceplate and a reflective surface adjacent the electron multiplier structure at the opposite end. In an image dissector tube,an accelerating mesh is disposed adjacent the photocathode, a scanning aperture plate at the entrance to the multiplierand a cylindrical anode positioned between the mesh and aperture plate, with the reflective surface within the anode. Other objects and advantages will become apparent from the following description in conjunction with the accompanying drawings.

- BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a schematic view in partial cross-section of a photomultiplier tube incorporating the reflective elements of the invention, and

FIG. 2 is a like view of an image dissector having the novel structure.

DESCRIPTION OF THE PREFERRED EMBODIMENTS As shown in FIG. 1, an evacuated tubular envelope I includes a transparent faceplate 12 at one end, pref-- erably of glass, which admits light radiation 14 from an external source. A photocathode 16, of a suitable material which emits electrons in response to light impingement thereon, is disposed on the inner surface of the 2 faceplate. A light reflective metallic layer 18, such as a thin coating of polished aluminum, is positioned directly on the glass underlying the photocathode. An additional layer of suitable transparent dielectric material 20 may be positioned between the reflective layer 18 and photocathode 16. The dielectric layer is of a specified thickness so that the reflective layer and photocathode are spaced apart a distance which is a predetermined wavelength of the light to provide interference enhancement. This is a well known effect wherein multiple light reflections are established within the dielectric layer between the reflective layer surface and photocathode surface to increase the electron emission efficiency of the photocathode. A suitable d.c. potential is applied to the photocathode 16 by a lead 22 connected through the tubular envelope to a conductive coating 24 on the faceplate. The conductive coating extends to the reflective layer 18 and over the dielectric layer 20 to the photocathode.

At the opposite end of the tube is, an electron multiplier structure including a plurality of successive secondary electron emissive coated. dynodes 26 having stepped voltages applied to provide an amplified electrical output signal at the output electrode 28. Adjacent the multiplier, intermediate the dynodes 26 and photocathode 16, is an electron focusing and accelerating structure 30 including a transverse support electrode 32, a cylindrical electrode'34 extending toward the faceplate and a conical electrode 36 having the narrow end facing the photocathode. A suitable d.c. potential isapplied to the electrode structure 30 including electrodes 32, 34 and 36 to focus and accelerate electrons from the photocathode along the path 38 through an opening 40 into the electron multiplier dynodes 26.

Around the cylindrical electrodes 34, adjacent the multiplier entrance, is a light reflective mirror 42 having a highly reflective smooth metal coating. The mirror preferably has a concave curvature which may be a spherical or parabolic surface that is capable of refleeting light 14 passing through the faceplate onto the mirror back toward the photocathode 16. The reflective layer 18 then projects the light into the photocathode.,The photocathode material is thus stimulated to emitelectrons in response to both the reflected light from reflective mirror 42 impinging directly on the outer photoemissive surface, as well as the reflected light from the underlying reflective layer 18 which penetrates the inner surface of the photocathode. The effect of the combined light reflections therefore provides increased light absorption in the photocathode and greater efficiency of electron emission. Still further photoemission enhancement is provided by the use of the intermediate dielectric layer 20 which establishes multiple light reflections on the photocathode.

As shown in FIG. 2, a similar light reflective arrange ment is employed in an image dissector tube of the type described in U.S. Pat. No. 3,295,010 issued Dec. 27, I966 and assigned to the same assignee as the instant application. In this case, an accelerating mesh grid electrode 44 is positioned adjacent the photocathode l6 and a tubular electrode 46 is disposed between the mesh 44 and the electron multiplier dynodes 26. The electrode 46 provides a uniform potential field free region in which electrons are focused and deflected without disturbance from accelerating electrostatic fields.

An apertured plate 48 is positioned at the opposite end provides a scanning aperture 49 for scanning electrons from successive portions of the photocathode which are directed into the multiplier. Suitable accelerating potentials are applied between the various electrodes, and magnetic focusing and deflection coils 50, 52 provide focusing and scanning of the electrons at the apertured plate 48.

The light reflective concave mirror 42 is now positioned within the tubular electrode 46 adjacent apertured plate 48 at the entrance to the multiplier. Incoming light 14 passes through faceplate. 12 around photocathode l6 and thorugh mesh 44 and tubular electrode 46 to strike mirror 42 which reflects the light back through mesh 44 onto photocathode 16. The underlying reflective layer 18, as in the previous configuration, redirects the reflected light into the inner surface of the photocathode while the light from mirror 42 strikes the outer photoemissive surface so that the combined reflections produce greater electron emission. An additional intermediate dielectric layer may also be used in this configuration if so desired. The mesh preferably has a relatively high transmission or coarse wire grid so as not to interfere with the passage of light therethrough. In another variation of this configuration, the mesh may be reduced in diameter and mounted on longer support wires 54 so that incoming light 14 will pass around the mesh into tubular electrode 46 and only reflected light from mirror 42 will pass through the mesh on the return path toward photocathode 16. This will eliminate some of the light interference caused by the mesh.

It may thus be seen that the present invention provides a multiplier phototube of improved sensitivity and electron emissive efficiency. While only specific embodiments have been described and illustrated, it is to be understood that other variations may be made in the particular design and configuration without departing from the scope of the invention as set forth in the appended claims.

What is claimed is:

1. An electron tube photomultiplier device comprisan evacuated tubular envelope having a light transparent faceplate at one end,

an electron multiplier at the opposite end,

a photocathode disposed on a central area of the inner surface of said faceplate emitting electrons in response to light impingement thereon, said faceplate passing light around said photocathode area,

a light reflective layer in said central area between said photocathode and faceplate,

an accelerating mesh spaced closely and parallel to said photocathode,

a tubular anode extending between said mesh and electron multiplier,

an apertured plate at the opposite end of said tubular anode providing a scanning aperture for electrons entering said electron multiplier,

light reflecting means positioned within said tubular anode adjacent said apertured plate and surrounding said aperture to reflect light from said faceplate through said mesh onto said photocathode and reflective layer,

means applying an electron accelerating field potential to said mesh, anode and electron multiplier with respect to said photocathode to direct electrons from said photocathode into said electron multiplier,

means for focusing electrons onto said apertured plate,

means for scanning electrons from successive areas of said photocathode across said aperture, and

output means connected to said electron multiplier.

2. The device of claim 1 wherein said photocathode includes a dielectric layer between said light reflective layer and photocathode, said dielectric layer spacing said photocathode from said reflective layer at a predetermined wavelength of light for providing interference enhancement.

3. The device of claim 1 wherein said light reflective means includes a concave apertured mirror.

4. The device of claim 3 wherein said accelerating mesh is of a smaller diameter than said tubular anode and includes an annular area between the diameters of said mesh and anode permitting light from said faceplate to pass into said anode around said mesh. 

1. An electron tube photomultiplier device comprising: an evacuated tubular envelope having a light transparent faceplate at one end, an electron multiplier at the opposite end, a photocathode disposed on a central area of the inner surface of said faceplate emitting electrons in response to light impingement thereon, said faceplate passing light around said photocathode area, a light reflective layer in said central area between said photocathode and faceplate, an accelerating mesh spaced closely and parallel to said photocathode, a tubular anode extending between said mesh and electron multiplier, an apertured plate at the opposite end of said tubular anode providing a scanning aperture for electrons entering said electron multiplier, light reflecting means positioned within said tubular anode adjacent said apertured plate and surrounding said aperture to reflect light from said faceplate through said mesh onto said photocathode and reflective layer, means applying an electron accelerating field potential to said mesh, anode and electron multiplier with respect to said photocathode to direct electrons from said photocathode into said electron multiplier, means for focusing electrons onto said apertured plate, means for scanning electrons from successive areas of said photocathode across said aperture, and output means connected to said electron multiplier.
 2. The device of claim 1 wherein said photocathode includes a dielectric layer between said light reflective layer and photocathode, said dielectric layer spacing said photocathode from said reflective layer at a predetermined wavelength of light for providing interference enhancement.
 3. The device of claim 1 wherein said light reflective means includes a concave apertured mirror.
 4. The device of claim 3 wherein said accelerating mesh is of a smaller diameter than said tubular anode and includes an annular area between the diameters of said mesh and anode permitting light from said faceplate to pass into said anode around said mesh. 